Method for providing asymmetric identification and access with respect to a radio-frequency tag

ABSTRACT

A method for providing identification and access with respect to a passive radio-frequency tag in a tag population, including, receiving a command for initiating an inventory round or a command for initiating a tag access, the command including, as a parameter, a number including an identifier of a reader, generating a 16-bit value referred to as first or second binding handle, the first or second binding handle including N juxtaposed bits forming the reader identifier, backscattering the first or second binding handle, receiving an ACK command, analyzing the identification parameter of the ACK command, and in a case where the identification parameter includes the reader identifier then ignoring the ACK command, only tags having the selected inventoried flag value for the session number are inventoried, the first initiating command only including the same inventoried flag for every inventoried tags during a session and wherein the at least other inventoried flag value is never used during the session.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to European Patent Application No.20212259.4 filed on Dec. 7, 2020, the entire disclosure of which ishereby incorporated herein by reference.

FIELD OF INVENTION

The invention relates to the field of radio-frequency identityprotocols, defining the physical and logical requirements forcommunications between passive transponders (tags) and readers. Thepresent invention relates particularly to a method for providingasymmetric identification and access with respect to a radio-frequencypassive tag.

In radio frequency identification, the term tag or transponder refers toa device configured to emit an identifying signal in response to aninterrogating received signal. A RFID tag does not necessarily have aninternal power supply or battery of its own, in this case the powerrequired to energize the transponder is obtained from an electromagneticfield generated by a reader, also known as interrogator, used tointerrogate the tag. Such a tag relies purely on backscattercommunication. The term passive refers to tags of this type.

BACKGROUND OF THE INVENTION

A radio-frequency identification (RFID) system comprises readers, alsoknown as interrogators, and tags, also known as labels or transponders.One or more RFID readers may communicate with one or more RFID tags inany numbers of ways. Some such ways are called protocols which call forspecific manners of signaling between readers and tags. EPC UHF Gen2 AirInterface Protocol is a widely used protocol, which defines physical andlogical requirements for a passive tag, in a case where a reader talksfirst, throughout the 860 to 960 MHz frequency range. The term “Gen2V2protocol” will be used to refer to this protocol in the subsequent text.

According to the Gen2V2 protocol, readers manage a tag population withintheir effective read zone using three basic operations. Each of theseoperations may comprise multiple commands. The operations are defined asfollows:

-   -   Select, to choose a tag population. The set of commands        dedicated to the Select operation includes the Select command.    -   Inventory, to identify individual tags within the tag        population. The set of commands dedicated to the Inventory        operation includes Query, QueryAdjust, QueryRep, ACK and NAK        commands.    -   Access, to communicate with an identified tag. The reader may        perform a core operation such as reading, writing, locking, or        killing the tag; a security-related operation such as        authenticating the tag; or a file-related operation such as        opening a particular file in the tag's memory. The set of        commands dedicated to the Access operation includes, among        others, Req_RN and Authenticate commands.

Readers support and tags provide 4 sessions, denoted S0, S1, S2 and S3.Tags participate in one and only one session during an inventory round,and two or more readers can use sessions to independently inventory acommon tag population. Each tag comprises four flags denoted‘inventoried flags’, each inventoried flag being associated with one ofthe four sessions. A tag shall maintain an independent inventoried flagfor each of its four sessions, and each inventoried flag may have twovalues, either value A or value B.

Moreover, each tag has a slot counter and a state, which may beimplemented all along the Select, Inventory and Access operations. Aslot counter contains a value, said value being used to determine thepoint in an inventory round at which a tag may respond, as it will beexplained later. A state characterizes the tag's behavior and responseto a reader command. The tag state set includes Ready, Arbitrate, Reply,Acknowledged, Open, Secured and Killed.

Ready is a holding state for energized tags that are neither Killed norcurrently participating in an Inventory round: upon entering anenergizing radio-frequency (RF) field, a tag that is not killed shallenter the Ready state. A reader may first issue a Select command toselect a population of tags in the Ready state. The Select command canset a tag's inventoried flag to either A or B in any one of the foursessions. In this case, the Select command comprises configurationparameters among which a session number and an inventoried flag value.The Select command also comprises selection criteria, which won't bedetailed here. Upon receiving a Select command, each tag evaluates theselection criteria, and depending on the evaluation may set theinventoried flag of the session specified in the command to the valuespecified in the command.

Subsequently, a reader shall issue a Query command. Query initiates anInventory round and decides which tags participate in the round. A Querycommand comprises a session number, an inventoried flag value, and aninteger in the range (0, 15), denoted slot-count parameter Q. Theslot-count parameter sets the number of slots in the inventory round.Upon receiving a Query command, tags with the matching inventoried flagvalue for the specified session shall draw a Q-bit random value fromtheir random number generator or pseudo-random number generator, andload said value into their slot counter. Subsequently, a tag shalltransition to the Arbitrate state and remain silent if the value in itsslot counter is nonzero, or transition to the Reply state if the valuein its slot counter is zero.

Arbitrate is a holding state for tags that are participating in thecurrent inventory round and whose slot counters hold nonzero values.After issuing a Query command but receiving no response within a time T1called ‘Immediate reply time’, the reader typically issues one or moreQueryAdjust or QueryRep commands. A QueryAdjust command comprises thesession number in the previous Query, but a higher or a smallerslot-count parameter Q. A tag in Arbitrate state shall adjust the valuein its slot counter upon receiving a QueryAdjust corresponding to theinventory round currently in progress, then pick a new Q-bit number, andload it into its slot counter. Subsequently, the tag shall transition tothe Reply state if said number is zero, or stay in the Arbitrate stateif said number is nonzero. A QueryRep command comprises the sessionnumber of the previous Query without changing the slot-count parameter.In the Arbitrate state, the tag shall decrement by one the value in itsslot counter every time it receives a QueryRep corresponding to theinventory round currently in progress, and it shall transition to theReply state when the value in its slot counter reaches zero. It shouldbe noted that a tag in the Acknowledged, Open, or Secured state thatreceives a QueryRep command whose session parameter matches the sessionparameter in the prior Query, and that is not in the middle of a Kill oran Access command sequence shall invert its inventoried flag (i.e. A→Bor B→A, as appropriate) for the current session and transition to Ready.

Upon entering the Reply state, the tag shall backscatter a 16-bit randomor pseudo-random number (called RN16). In response, the reader shallacknowledge the tag by echoing the tag's backscattered RN16. This shallbe done by sending an ACK command comprising the RN16. Thus, if the tagreceives an ACK command containing an identical RN16 (a valid ACKcommand), it shall transition to the Acknowledged state. Subsequently,the tag shall send an EPC code, stored in a part of its memory called‘EPC memory’. The EPC code identifies the object to which the Tag is orwill be attached. It should be noted that another ACK command comprisingthe RN16 may be sent again to the tag in said Acknowledged state, so asto receive the EPC code again. On the contrary, if the tag fails toreceive an ACK command within a time T2 or receives an invalid ACK (thatis to say an ACK command including a different RN16), then it shallreturn to the Arbitrate state.

Access always begins with a reader moving a tag from the Acknowledgedstate to either the Open or the Secured state by issuing a Req_RNcommand comprising said tag's RN16. If the tag in the Acknowledged statereceives a Req_RN with a correct RN16, it shall generate, store andbackscatter a new 16-bit random or pseudo-random number, denoted handle,and transition to the Open or Secured state. Subsequently, if the readerwants to ensure that only this tag is in the Open or Secured state, thenit may issue an ACK command with said handle as a parameter. The tagthat receives the ACK command with the correct handle shall reply bybackscattering said handle and remain in its current state (Open orSecured, as appropriate), whereas those that receive an ACK command withan incorrect handle shall transition from the Open or Secured state tothe Arbitrate state.

The choice of transitioning to the Open or to the Secured state whenreceiving a Req_RN command with a correct handle depends on the tag'saccess password, said access password being a value stored in a memoryof the tag. It should be noted that a tag in the Open state may executesome commands only, whereas a tag in the Secured state with appropriateprivileges may execute all commands. A tag in the Open state shalltransition to the Secured state after a successful authentication. Inorder to initiate said authentication, a reader may issue anAuthenticate command, said command comprising the tag's handle.

A reader and a tag can communicate indefinitely in the Open or Securedstate, but the reader may end the communication at any time in order toidentify and access other tags by issuing a QueryRep command. In thiscase, the tag shall transition from the Open or Secured state to theReady state. The whole process ends when the reader has issued Q−1QueryRep commands subsequent to a Query command, where Q is theslot-count parameter comprised in said Query command.

FIG. 1 shows a RFID system comprising two readers, Reader E and ReaderG, and a Tag population comprising five Tags 1, 2, 3, 4, 5. Tags 1, 2and 3 are in the effective read zone for Reader E, and Tags 3, 4 and 5are in the effective read zone for Reader G. This means Tags 1, 2 and 3can receive operating energy and commands, and backscatter responsesfrom a RF field generated by Reader E, and Tags 3, 4 and 5 can receiveoperating energy and commands, and backscatter responses from a RF fieldgenerated by Reader G.

By way of example only, FIGS. 2A and 2B illustrate an inventory andaccess sequence between Reader E and Tags 1, 2 and 3 according to theGen2V2 protocol. First column relates to Reader E, and shows inparticular commands that Reader E issues, and responses that Reader Ereceives during the communication sequence. Last column relates to theTags, and in particular shows actions performed by a Tag in response toa command received from Reader E. The middle column shows messagesdirections: an arrow pointing right indicates a command from Reader E toat least a Tag; an arrow pointing left indicates a response from atleast a tag to Reader E. The second column shows the communication slot,which starts from zero and is incremented by one every time Reader Eissues a QueryRep command subsequently to a Query command. When thecommunication slot reaches Q−1, where Q is the slot-count parameterincluded in the Query command, the sequence ends. The fourth columnshows timing requirements to be applied to a command or a reply.

FIGS. 3A and 3B—resp. 4A-5B—relates to Tag 1—resp. Tag 2, 3. For eachtable, first column shows the state of the Tag, second column shows thevalue in the slot counter of the Tag, third column shows the RN16 or thehandle (in hexadecimal) generated by the random number generator orpseudo-random number generator of the Tag, fourth column shows thesession used by the Tag to communicate with Reader E, and last columnshows the inventoried flag value associated with said session.

In a step 1, Reader E generates an RF field. As a result, Tags 1, 2 and3, which are in the read-zone for Reader E, enter the Ready state. Atthis stage, for each of these Tags, the slot counter does not containany value, no RN16 or handle has been generated yet, the session to beused with Reader E has not been defined yet, and the inventoried flagvalue is not applicable since the session is undefined.

In a step 2, Reader E issues a Select command, said command comprising asession number Sx and an inventoried flag value IFVx. In the describedexample, the session number Sx is S2 and the inventoried flag value IFVxis A. As a result, Tags 1, 2 and 3 set their inventoried flag value forsession S2 to A.

In a step 3, Reader E issues a Query command, said command comprising asession number Sx, an inventoried flag value IFVx, and a slot-countparameter Q in the range (0, 15). In the described example, the sessionnumber Sx is S2, the inventoried flag value IFVx is A, and theslot-count parameter Q is 8. As a result, Tag 1, 2 and 3 enter theArbitrate state, and generate a Q-bit number and a 16-bit number (RN16)using their random number generator or pseudo-random number generator.In the described example, the Q-bit number and the RN16 generated by Tag1 are 1 and 1111; the Q-bit number and the RN16 generated by Tag 2 are 2and 2222; and the Q-bit number and the RN16 generated by Tag 3 are 3 and3333. The communication slot is set to zero.

In a step 4, Reader E waits for time T1 and does not receive any reply,since none of Tags 1, 2 or 3 has zero as a value in their slot counter.

In a step 5, Reader E issues a QueryRep command, said command comprisingthe session number of the previous Query command, that is to say S2. Asa result, Tags 1, 2 and 3, whose slot counters hold nonzero values,decrement their slot counter. Moreover, since the value in Tag 1's slotcounter turns zero, Tag 1 transitions to the Reply state. Thecommunication slot is increased to one.

In a step 6, Tag 1 backscatters its RN16, said RN16 being received byReader E.

In a step 7, Reader E acknowledges Tag 1 by echoing its RN16 within anACK command. As a result, Tag 1 transitions to the Acknowledged state.

In a step 8, Tag 1 sends its EPC code, said EPC code being received byReader E.

In a step 9, Reader E sends a Req_RN command comprising Tag 1's RN16,which makes Tag 1 transition to the Open or Secured state, depending onTag 1's access password.

In a step 10, Tag 1 generates, stores and backscatters a new 16-bitrandom or pseudo-random number, the handle, said handle being receivedby Reader E. In the described example, the handle generated by Tag 1 is6666.

In a step 11, Reader E sends an Authenticate command comprising Tag 1'shandle. As a result, Tag 1 transitions to the Open state.

In a step 12, Tag 1 backscatters a crypto response, said crypto responsebeing received by Reader E.

In a step 13, Reader E issues a QueryRep command so as to identify otherTags. Said command comprises the session number of the previous Querycommand, that is to say S2. As a result, Tag 1 inverts its inventoriedflag (A→B) and transitions to the Ready state. Besides that, all Tagswith inventoried flag value A for session S2 and whose slot counterholds a nonzero value, that is to say Tags 2 and 3, decrement their slotcounter. Since the value in Tag 2's slot counter turns zero, Tag 2transitions to the Reply state. The communication slot is increased totwo.

In a step 14, Tag 2 backscatters its RN16, said RN16 being received byReader E.

In a step 15, Reader E acknowledges Tag 2 by echoing its RN16 within anACK command. As a result, Tag 2 transitions to the Acknowledged state.

In a step 16, Tag 2 sends its EPC code, said EPC code being received byReader E.

In a step 17, Reader E sends a Req_RN command comprising Tag 2's RN16,which makes Tag 2 transition to the Open or Secured state, depending onTag 2's access password.

In a step 18, Tag 2 generates, stores and backscatters a new 16-bitrandom or pseudo-random number, the handle, said handle being receivedby Reader E. In the described example, the handle generated by Tag 2 is7777.

In a step 19, Reader E sends an Authenticate command comprising Tag 2'shandle. As a result, Tag 2 transitions to the Open state.

In a step 20, Tag 2 backscatters a crypto response, said crypto responsebeing received by Reader E.

In a step 21, Reader E issues a QueryRep command so as to identify otherTags. Said command comprises the session number of the previous Querycommand, that is to say S2. As a result, Tag 2 inverts its inventoriedflag (A→B) and transitions to the Ready state. Besides that, all Tagswith inventoried flag value A for session S2 and whose slot counterholds a nonzero value, that is to say Tag 3 only, decrement their slotcounter. Since the value in Tag 3's slot counter turns zero, Tag 3transitions to the Reply state. The communication slot is increased tothree.

In a step 22, Tag 3 backscatters its RN16, said RN16 being received byReader E.

In a step 23, Reader E acknowledges Tag 3 by echoing its RN16 within anACK command. As a result, Tag 3 transitions to the Acknowledged state.

In a step 24, Tag 3 sends its EPC code, said EPC code being received byReader E.

In a step 25, Reader E sends a Req_RN command comprising Tag 3's RN16,which makes Tag 3 transition to the Open or Secured state, depending onTag 3's access password.

In a step 26, Tag 3 generates, stores and backscatters a new 16-bitrandom or pseudo-random number, the handle, said handle being receivedby Reader E. In the described example, the handle generated by Tag 3 is8888.

In a step 27, Reader E sends an Authenticate command comprising Tag 3'shandle. As a result, Tag 3 transitions to the Open state.

In a step 28, Tag 3 backscatters a crypto response, said crypto responsebeing received by Reader E.

In a step 29, Reader E issues a QueryRep command so as to identify otherTags. Said command comprises the session number of the previous Querycommand, that is to say S2. As a result, Tag 3 inverts its inventoriedflag (A→B) and transitions to the Ready state. Nothing else happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to four.

In a step 30, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command.

In a step 31, Reader E issues another QueryRep comprising the sessionnumber of the previous Query command, that is to say S2. Nothing happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to five.

In a step 32, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command.

In a step 33, Reader E issues another QueryRep comprising the sessionnumber of the previous Query command, that is to say S2. Nothing happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to six.

In a step 34, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command.

In a step 35, Reader E issues another QueryRep comprising the sessionnumber of the previous Query command, that is to say S2. Nothing happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to seven.

In a step 36, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command. Since the slot-count parameter is 8and the communication slot has reached 7, the inventory round is endedafter this last unsuccessful attempt to identify other Tags.

As already explained, all ACK commands issued by a Reader End intendedfor a tag shall include a 16-bit random number previously generated andbackscattered by the tag, said number being either the RN16 or thehandle. If a reader issues an ACK command to a tag in the Reply orAcknowledged state, then the echoed number shall be the RN16 that thetag previously backscattered as it transitioned from the Arbitrate stateto the Reply state. If the reader issues an ACK command to a tag in theOpen or Secured state, then the echoed number shall be the tag's handle.For reasons of convenience, the echoed 16-bit random number (either theRN16 or the handle) included in an ACK command is referred to as‘session handle’. As a consequence, ‘session handle’ may referindifferently to a tag's RN16 or handle. Upon receiving an ACK command,a tag shall verify that the session handle is correct prior to executingsaid command.

Receiving an ACK command with an incorrect session handle may happen ina RFID system having multiple RFID readers operating in parallel,because some tags might be simultaneously within the effective read zonefor more than one reader, whether this is intentional or not. Forinstance, in the example illustrated in FIG. 1 , Tag 3 is in theeffective read zone for both Readers A and B. Thus, Tag 3 may hear anACK command issued by Reader G while being in communication with ReaderE. Said ACK command may for instance be intended for Tag 4. Theprobability of two readers simultaneously communicating with two tagswhich have the same session handle being 1 in 2¹⁶, the session handleincluded in said ACK command is likely to be different from Tag 3'ssession handle.

A tag hearing an ACK command with an incorrect session handle shall notexecute said command. Furthermore, as already mentioned, if said tagstate is Open or Secured, then the tag shall transition to the Arbitratestate. This is problematic because a transition to the Arbitrate stateessentially terminates communication between the tag and the reader withwhich the tag was communicating. In the previous-mentioned example, ifTag 3 is in the Open or the Secured state and hears Reader G sending anACK command intended to Tag 4, said ACK command comprising Tag 4'ssession handle, then Tag 3 shall transition to the Arbitrate state,thereby losing connection with Reader E.

If a tag terminates communication with a reader, there is usually thepossibility to identify and access said tag at a later time. However,this is not always the case when the tag is moving rapidly through readzones.

Furthermore, the possibility to use two or more inventoried flags couldinduce mistakes if a tag having a wrong inventoried flag, answers to thereader during a session or if a tag changes his state while he shouldkeep the same state. The multiplicity of inventoried flags is not safeenough, in particular if a Reader sends wrong inventoried flags to oneor several Tags.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an asymmetric identificationand access method, compatible with the Gen2V2 protocol, whereininterferences between various inventoried flags during the same sessionare avoided. It is another object of the invention to provide anasymmetric identification and access method wherein cases ofcommunication loss due to receiving an ACK command with an incorrectsession handle are avoided.

Thus, the invention concerns a method for providing asymmetricidentification and access with respect to a passive radio-frequency tagin a tag population, the passive tag being configured for receivingcommands, among which commands for acknowledging a tag, referred to asACK, an ACK command comprising an identification parameter in the formof a 16-bit number, the tag having:

-   -   a slot counter holding a value determining the point in an        inventory round at which the tag may respond    -   a random number generator or pseudo-random number generator,        arranged to generate a 16-bit value when the tag starts        participating in an inventory round, and arranged to generate        another 16-bit value when the reader starts accessing the tag    -   a state that characterizes the tag's behavior and response to        reader's commands, said state being part of a state list        comprising:        -   Ready, which is a holding state for a tag not currently            participating in an inventory round        -   Arbitrate, which is holding state for a tag currently            participating in an inventory round and whose slot counter            holds a nonzero value        -   Open or Secured, which are states a tag transitions to when            a reader starts accessing it.

According to the invention, the method comprises the following steps,performed by the tag initially in the Ready state:

-   -   receiving a command for initiating an inventory round, referred        to as inventory initiating command, said inventory initiating        command comprising, as a parameter, a reader identifier in the        form of a N-bit number, N being an integer in the range (1, 15)    -   generating, by means of the random number generator or        pseudo-random number generator, a 16-bit value referred to as        first binding handle, said first binding handle comprising N        juxtaposed bits forming the reader identifier    -   backscattering the first binding handle, said first binding        handle being received by the reader    -   receiving a command for initiating a tag access, referred to as        access initiating command, said access initiating command having        the first binding handle as a parameter    -   generating, by means of the random number generator or        pseudo-random number generator, a 16-bit value referred to as        second binding handle, said second binding handle comprising N        juxtaposed bits forming the reader identifier, said reader        identifier being located within the second binding handle at a        location similar to the location of the reader identifier within        the first binding handle    -   backscattering the second binding handle, said second binding        handle being received by the reader    -   transitioning to the Open or Secured state    -   receiving an ACK command    -   comparing the reader identifier with a code formed by N bits of        the identification parameter of said ACK command that are        positioned at a location similar to the location of the reader        identifier within the first or second binding handle, and in a        case where said code and said reader identifier differ, then        ignoring said ACK command,

the tag having an inventoried flag for each session a reader may use toidentify the tag within an inventory round, said inventoried flag havinga value indicating whether the tag may respond to the reader, theinventoried flag comprising at least two possible values, A or B forexample, the first initiating command including a session number as aparameter, and one selected inventoried flag value of the at least twopossible values, the method comprising the following step, performedafter receiving the inventory initiating command: setting theinventoried flag to the selected value for the session specified in theinventory initiating command.

According to the invention, only tags having the selected inventoriedflag value for the session number are inventoried, the first initiatingcommand only including the same inventoried flag for every inventoriedtags during a session, and the at least other inventoried flag value isnever used by the reader during said session.

The invention offers a new command, called inventory initiating command,which may be integrated within the Gen2v2 protocol. Preferentially, theinventory initiating command provides similar functionality to thecommand sequence Select/Query as defined in the Gen2V2 protocol. Theinventory initiating command enables the reader to start an inventoryround, and instructs the tag to participate in said inventory round.Upon receiving the inventory initiating command, the tag shall generatea first binding handle, as it would have generated a RN16 if it had beenin response to a Query, QueryAjust or QueryRep command. However, thefirst binding handle differs from a classic RN16 in that it is notcompletely a random number: a portion of the first binding handle ismade of the reader identifier included in the inventory initiatingcommand. The identifier for the reader may be a unique value that isdifferent from the values for all other readers, or at least for readersthat have some portion of their reading zone in common. Then, the tagshall backscatter the first binding handle, as it would have done with aclassic RN16. Subsequently, the first binding handle is received by thereader, and said first binding handle may be used to identify andacknowledge the tag by using an ACK command whose identificationparameter is the first binding handle.

Then, at some point, the reader sends the access initiating command,namely a Req_RN command as defined in the Gen2V2 protocol, so as toinitiate an Access operation with the tag. As defined by the Gen2V2, theReq_RN shall comprise the 16-bit value previously backscattered by thetag, which is, in this case, the first binding handle. Upon receivingthe access initiating command comprising the first binding handle, thetag shall generate a second binding handle. The second binding handlediffers from a classic handle in that it is not completely a randomnumber: a portion of the second binding handle is made of the readeridentifier included in the inventory initiating command. Then, the tagshall backscatter the second binding handle, as it would have done witha classic handle, and transition to the Open or Secured state.Subsequently, the second binding handle is received by the Reader Endthe Access operation may go on.

From this point and as long as the tag is in the Open or Secured state,if the reader issues an ACK command intended for the tag, said ACKcommand shall have the second binding handle as an identificationparameter. According to the Gen2V2 protocol, a tag in the Open orSecured state shall transition to the Arbitrate state upon receiving anACK command whose identification parameter is different from the tag'shandle. According to the invention, a tag which has received a inventoryinitiating command earlier in the identification and access sequence andis now in the Open or Secured state, shall, upon receiving an ACKcommand, determine whether or not the identification parameter of saidACK command comprises the reader identifier of the first identificationcommand. If not, it means that the ACK command has been issued byanother reader. Then the tag simply ignores the ACK command and does nottransition to the Arbitrate state. It should be noted that if this otherreader uses the method according to the invention as well to identifyand access another tag, then the identification parameter in the ACKcommand comprises the other reader's identifier. Otherwise, theidentification parameter in the ACK command is a complete random number.

In addition, by using only one inventoried flag during the same session,interferences between tags are avoided. Indeed, the reader is only ableto inventory and send commands to tags with always the same inventoriedflag, so that a wrong inventoried flag can't be received by a tag thatwould induces mistakes in the tag behavior. The word “assymetric” meansthat an inventory or a command sent by the reader is only possible withone inventoried flag, even if tags have another selectable inventoriedflag. The reader is configured to use only one inventoried flag in asingle session. Preferably, the reader uses the same inventoried flagduring every sessions.

On the contrary, the Gen2v2 protocol allows the reader to use bothinventoried flags A and B in the same session. Thus, the readerinventories a first group of tags having the inventoried flags A, andanother group of tags having the inventoried flag B, for example. Thisconfiguration is named “symmetric” as a reader is able to orderindependently the same command to the first group of tags, and to thesecond group of tags in an independent manner. Of course, this protocolis not safe enough to avoid mistakes due to interferences betweeninventoried flags A and B.

Furthermore, the method according to the invention may comprise one or acombination of the following characteristics.

In an embodiment, in a case where said code and said reader identifiermatch, then the method comprises a step of comparing the other bits ofthe parameter with the other bits of the second binding handle, and in acase where said bits match, then the method comprises a step ofresponding to the ACK command.

In an embodiment, the state list comprises Reply, which is a state a tagcurrently participating in an inventory round transitions to when thevalue in its slot counter becomes zero, the random number generator orpseudo-random number generator is arranged to generate a Q-bit valuewhen the tag starts participating in an inventory round, Q being aninteger in the range (0, 15), the inventory initiating command comprisesQ as a parameter, said parameter being referred to as slot-countparameter, and the method comprises the following step, performed afterreceiving the inventory initiating command:

-   -   generating, by means of the random number generator or        pseudo-random number generator, a Q-bit value and loading it        into the slot counter    -   transitioning to the Arbitrate state if said Q-bit value is        nonzero, or to the Reply state otherwise.

It should be noted that including the Q-bit value in the first andsecond binding handles provides additional confidence to the reader thata tag response is for the correct reader.

In an embodiment, the first binding handle and the second binding handlecomprise Q juxtaposed bits different from the N juxtaposed bits, said Qjuxtaposed bits forming the Q-bit value, the Q-bit value beingpositioned within the first binding handle at a location similar to thelocation of the Q-bit value within the second binding handle.

In an embodiment, Q is in the range 0 to 5.

In an embodiment, the 16-N-Q bits left in the first and second bindinghandles are generated randomly by the random number generator orpseudo-random number generator.

In an embodiment, the selected inventoried flag is always A for eachsession.

In an embodiment, the reader identifier is a 5-bit number, that is tosay N=5.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described subsequently with reference tothe attached drawings, given by way of example, but in no way limitedthereto, in which:

FIG. 1 , already mentioned, is a schematic representation of a RFIDsystem comprising two readers, Reader E and Reader G, and a Tagpopulation comprising five Tags 1, 2, 3, 4, 5, wherein Tag 3 is in theeffective read zone for both Readers A and Reader G;

FIGS. 2A and 2B, already mentioned, illustrates an inventory and accesssequence between Reader E and Tags 1, 2 and 3;

FIGS. 3A, 3B, 4A, 4B, 5A, and 5B illustrate some features of Tags 1, 2and 3 along the communication sequence of FIG. 2 ;

FIGS. 6A, 6B, and 6C illustrate an inventory and access sequence betweenReader E and Tags 1, 2 and 3, according to an embodiment of the presentinvention;

FIGS. 7A, 7B, 8A, 8B, 9A, and 9B illustrate some features of Tags 1, 2and 3 along the communication sequence of FIGS. 6A-6C.

DETAILED DESCRIPTION OF THE INVENTION

The method according to the invention is supposed to be carried out by aradio-frequency passive tag. Said tag comprises means to carry out theGen2V2 protocol, so it has, as it has been explained before in moredetails:

-   -   a memory in which an EPC code and an access password are stored    -   four inventoried flags, one for each session S1, S2, S3, S4 the        reader may use to identify the tag within an inventory round.        Each inventoried flag has either value A or value B, said value        indicating whether the tag may respond to the reader.    -   a slot counter holding a value determining the point, in an        inventory round initiated by the reader, at which the tag may        respond.    -   a random number generator or pseudo-random number generator,        arranged to generate:        -   a 16-bit value referred to as RN16, when it starts            participating in an inventory round, that is to say in            response to a Query, QueryRep or QueryAdjust command with            appropriate parameters        -   a Q-bit value, Q being an integer in the range (0, 15), when            it starts participating in an inventory round, that is to            say in response to a Query, QueryRep or QueryAdjust with            appropriate parameters        -   a 16-bit value referred to as handle, when it starts            participating in an access operation, that is to say in            response to a Req_RN command having the RN16 as a parameter.

As defined in the Gen2V2 protocol, the tag's behavior and response to areader's command is defined by its state, which are, among others:

-   -   Ready, which is a holding state for a tag not currently        participating in an inventory round    -   Arbitrate, which is holding state for a tag participating in an        inventory round and whose slot counter holds a nonzero value    -   Reply, which is a state a tag in the Arbitrate state transitions        to when the value in its slot counter reaches zero    -   Acknowledged, which is a state a tag in the Reply state        transitions to when it receives an ACK command whose        identification parameter is identical to its RN16    -   Open or Secured, which are states a tag in the Acknowledged        state transitions to when it receives an Req_RN command whose        parameter is identical to its RN16.

According to the invention, an additional command is added to the set ofcommands defined in the Gen2V2 protocol. This command, referred to asinventory initiating command, provides similar functionality to thecommand sequence Select/Query. The inventory initiating command has thefollowing parameters:

-   -   a session number Sx (S1, S2, S3 or S4)    -   only one inventoried flag value IFVx for a single session        (preferably A), so that only tags having the selected        inventoried flag value for the session number are inventoried,        the other inventoried flag (B) value is never used during the        session    -   a slot-count parameter Q, Q being an integer in the range (0,        15)    -   an identifier RD of the reader that issues the inventory        initiating command, in the form of a N-bit value, N being an        integer in the range (1, 15).

Upon receipt of an inventory initiating command while in the Readystate, the tag shall:

-   -   set its inventoried flag (A) for the session Sx instructed in        the inventory initiating command, to the value IFVx specified in        the inventory initiating command,    -   generate, by means of the random number generator or        pseudo-random number generator, a Q-bit value and loading it        into the slot counter    -   transition to the Arbitrate state if said Q-bit value is        nonzero, or to the Reply state otherwise    -   generate, by means of the random number generator or        pseudo-random number generator, a 16-bit value referred to as        first binding handle, said first binding handle comprising N        juxtaposed bits forming the reader identifier RD specified in        the inventory initiating command, Q juxtaposed bits forming the        Q-bit value, and 16-N-Q random bits. Preferably, N=5 and Q is in        the range 0 to 5. In the described embodiment, the binding        handle has the following structure: the 4 most significant bits        represent the reader identifier, the 4 least significant bits        represent the Q-bit value, and the 8 middle bits are a random        value. In another embodiment, the first binding handle comprises        N juxtaposed bits, preferably 4 or 5, forming the reader        identifier RD specified in the inventory initiating command, and        16-N random bits. In this other embodiment, the binding handle        has preferably the following structure: the 4 or 5 most        significant bits represent the reader identifier, and the 12 or        11 least important bits are a random value.    -   backscatter the first binding handle.

A tag that has previously received a first initial command within a sameinventory and access sequence shall, upon receipt of a Req_RN commandwhile in the Acknowledged state, in a case where said Req_RN has thetag's first binding handle as a parameter:

-   -   transition to the Open or Secured state, the choice of ending        state depending on the Tag's access password (in the access        password is zero, then the tag shall transition to the Secured        state; if the access password is nonzero, then the tag shall        transition to the Open state)    -   generate, by means of the random number generator or        pseudo-random number generator, a 16-bit value referred to as        second binding handle, said second binding handle having the        same structure as the first binding handle: for instance the 4        most significant bits represent the reader identifier, the 4        least significant bits represent the Q-bit value, and the 8        middle bits are a random value.    -   backscatter the second binding handle.

By way of example only, FIGS. 6A-6C illustrate an inventory and accesssequence between Reader E and Tags 1, 2 and 3, implementing theinventory initiating command. First column relates to Reader E, andshows in particular commands Reader E issues, and responses Reader Ereceives during the sequence. Last column relates to the Tags, and inparticular shows actions performed by a Tag in response to a commandfrom Reader E. The middle column shows messages directions: an arrowpointing right indicates a command from Reader E to at least a Tag; anarrow pointing left indicates a response from at least a tag to ReaderE. The second column shows the communication slot, which starts fromzero and is incremented by one every time Reader E issues a QueryRepcommand subsequently to a inventory initiating command. When thecommunication slot reaches Q−1, the sequence ends. The fourth columnshows timing requirements to be applied to a command or a reply.

FIGS. 7A and 7B—resp. 8A-9B—relates to Tag 1—resp. Tag 2, 3. For eachtable, first column shows the state of the Tag, second column shows thevalue in the slot counter of the Tag, third column shows a first or asecond binding handle (in hexadecimal) generated by the random numbergenerator or pseudo-random number generator of the Tag at some points ofthe sequence, fourth column shows the session used by the Tag tocommunicate with Reader E, and last column shows the inventoried flagvalue associated with said session.

In a step 1, Reader E generates an RF field. As a result, Tags 1, 2 and3, which are in the read zone for Reader E, enter the Ready state. Atthis stage, for each of these Tags, the slot counter does not containany value, no first or second binding handle has been generated yet, thesession to be used with Reader E has not been defined yet, and theinventoried flag value is not applicable since the session is undefined.

In a step 2, Reader E issues a first initiation command having asparameters:

-   -   the session number S2    -   the only one inventoried flag value (preferably A)    -   8 as a slot-count parameter Q    -   the reader identifier C (in hexadecimal, that is to say 1100 in        the binary system).

As a result, Tags 1, 2 and 3 set their inventoried flag value forsession S2 to A. In addition, Tags 1, 2 and 3 generate a 8-bit numberand a 16-bit number (first binding handle) using their random numbergenerator or pseudo-random number generator. In the describedembodiment, the binding handles have the following structure: the 4 mostsignificant bits represent the reader identifier, the 4 leastsignificant bits represent the Q-bit value, and the 8 middle bits are arandom value. In the illustrated example, the 8-bit number and the firstbinding handle generated by Tag 1 are 1 and C661; the 8-bit number andthe first binding handle generated by Tag 2 are 2 and C772; and the8-bit number and the first binding handle generated by Tag 3 are 3 andC883. Moreover, Tag 1, 2 and 3 enter the Arbitrate state. Besides that,the communication slot is set to zero.

In a step 3, Reader E waits for time T1 and does not receive any reply,since none of Tags 1, 2 or 3 has zero as a value in their slot counter.

In a step 4, Reader E issues a QueryRep command, said command comprisingthe session number of the previous Query command, that is to say S2. Asa result, Tags 1, 2 and 3, whose slot counters hold nonzero values,decrement their slot counter. Moreover, since the value in Tag 1's slotcounter turns zero, Tag 1 transitions to the Reply state. Thecommunication slot is increased to one.

In a step 5, Tag 1 backscatters its first binding handle, said firstbinding handle being received by Reader E.

In a step 6, Reader E acknowledges Tag 1 by echoing the first bindinghandle within an ACK command. As a result, Tag 1 transitions to theAcknowledged state.

In a step 7, Tag 1 sends its EPC code, said EPC code being received byReader E.

In a step 8, Reader E sends a Req_RN command comprising Tag 1's firstbinding handle, which makes Tag 1 transition to the Open or Securedstate, depending on Tag 1's access password.

In a step 9, Tag 1 generates, stores and backscatters a new 16-bitrandom or pseudo-random number, the second binding handle, said secondbinding handle being received by Reader E. In the described example, thesecond binding handle generated by Tag 1 is CDD1.

In a step 10, Reader E sends an Authenticate command comprising Tag 1'ssecond binding handle. As a result, Tag 1 transitions to the Open state.

In a step 11, Tag 1 backscatters a Crypto response, said Crypto responsebeing received by Reader E.

In a step 12, Reader E issues a QueryRep command so as to identify otherTags. Said command comprises the session number of the previous Querycommand, that is to say S2. As a result, Tag 1 inverts its inventoriedflag (A→B) and transitions to the Ready state. Besides that, the otherTags having the inventoried flag value A for session S2 and whose slotcounter holds a nonzero value, that is to say Tags 2 and 3, decrementtheir slot counter. Since the value in Tag 2's slot counter turns zero,Tag 2 transitions to the Reply state. The communication slot isincreased to two.

In a step 13, Tag 2 backscatters its first binding handle, said firstbinding handle being received by Reader E.

In a step 14, Reader E acknowledges Tag 2 by echoing the first bindinghandle within an ACK command. As a result, Tag 2 transitions to theAcknowledged state.

In a step 15, Tag 2 sends its EPC code, said EPC code being received byReader E.

In a step 16, Reader E sends a Req_RN command comprising Tag 2's firstbinding handle, which makes Tag 2 transition to the Open or Securedstate, depending on Tag 2's access password.

In a step 17, Tag 2 generates, stores and backscatters a new 16-bitrandom or pseudo-random number, the second binding handle, said secondbinding handle being received by Reader E. In the described example, thesecond binding handle generated by Tag 2 is CEE2.

In a step 18, Reader E sends an Authenticate command comprising Tag 2'ssecond binding handle. As a result, Tag 2 transitions to the Open state.

In a step 19, Tag 2 backscatters a crypto response, said crypto responsebeing received by Reader E.

In a step 20, Reader E issues a QueryRep command so as to identify otherTags. Said command comprises the session number of the previous Querycommand, that is to say S2. As a result, Tag 2 inverts its inventoriedflag (A→B) and transitions to the Ready state. Besides that, all Tagswith inventoried flag value A for session S2 and whose slot counterholds a nonzero value, that is to say Tag 3 only, decrement their slotcounter. Since the value in Tag 3's slot counter turns zero, Tag 3transitions to the Reply state. The communication slot is increased tothree.

In a step 21, Tag 3 backscatters its first binding handle, said firstbinding handle being received by Reader E.

In a step 22, Reader E acknowledges Tag 3 by echoing its first bindinghandle within an ACK command. As a result, Tag 3 transitions to theAcknowledged state.

In a step 23, Tag 3 sends its EPC code, said EPC code being received byReader E.

In a step 24, Reader E sends a Req_RN command comprising Tag 3's firstbinding handle, which makes Tag 3 transition to the Open or Securedstate, depending on Tag 3's access password.

In a step 25, Tag 3 generates, stores and backscatters a new 16-bitrandom or pseudo-random number, the second binding handle, said secondbinding handle being received by Reader E. In the described example, thehandle generated by Tag 3 is CFF3.

In a step 26, Reader E sends an Authenticate command comprising Tag 3'shandle. As a result, Tag 3 transitions to the Open state.

In a step 27, Tag 3 backscatters a Crypto response, said Crypto responsebeing received by Reader E.

In a step 28, Reader E issues a QueryRep command so as to identify otherTags. Said command comprises the session number of the previous Querycommand, that is to say S2. As a result, Tag 3 inverts its inventoriedflag (A→B) and transitions to the Ready state. Nothing else happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to four.

In a step 29, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command.

In a step 30, Reader E issues another QueryRep comprising the sessionnumber of the previous Query command, that is to say S2. Nothing happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to five.

In a step 31, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command.

In a step 32, Reader E issues another QueryRep comprising the sessionnumber of the previous Query command, that is to say S2. Nothing happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to six.

In a step 33, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command.

In a step 34, Reader E issues another QueryRep comprising the sessionnumber of the previous Query command, that is to say S2. Nothing happenssince there are no more tags in Reader E's read zone that have A as aninventoried flag value for session S2. The communication slot isincreased to seven.

In a step 35, Reader E waits for time T1 and does not receive any replyto the previous QueryRep command. Since the slot-count parameter is 8and the communication slot has reached 7, the inventory round is endedafter this last unsuccessful attempt to identify other Tags.

As already mentioned, Tag 3 is in the read zone for both Reader E andReader G, which implies that Tag 3 may receive, at any time of anidentification and access sequence according to the invention, an ACKcommand issued by Reader G and intended for another tag, let us say Tag4. Thus, according to the invention, Tag 3 analyzes the bits of theidentification parameter of the ACK command where Tag 3's identifier issupposed to be positioned (in the preferred embodiment, the 4 mostsignificant bits of said identification parameter), so as to determinewhich reader has issued said command. Of course, Reader G may use withTag 4 an identification and access sequence according to the invention(that is to say a sequence comprising a inventory initiating command),or a classic inventory and access sequence according to the Gen2V2protocol (that is to say a sequence comprising a Select and a Querycommand). In the first case, the identification parameter included inthe ACK command is Tag 4's first or second binding handle, said first orsecond binding handle comprising Tag 4's identifier. Thus, the bits ofthe identification parameter of the ACK command where Tag 3's identifieris supposed to be positioned form Tag 4's identifier instead, and Tag 3concludes the ACK command is invalid. In the second case, theidentification parameter included in the ACK command is a 16-bit randomnumber, either Tag 4's RN16 or Tag 4's handle. Thus, the bits of theidentification parameter of the ACK command where Tag 3's identifier issupposed to be do not form any reader identifier and Tag 3 concludes theACK command is invalid.

When Tag 3, performing with Reader E an identification and accesssequence according to the invention, receives an ACK command issued byReader G:

-   -   if Tag 3's state is not Open or Secured, then Tag 3 ignores ACK        command from Reader G (as defined by the Gen2V2 protocol)    -   if Tag 3's state is Open or Secured, the identification        parameter do have the correct reader identifier, but the        identification parameter differs from Tag 3's second binding        handle, then Tag 3 will transition to Arbitrate and stop        communication with Reader E (as defined by the Gen2V2 protocol)    -   if Tag 3's state is Open or Secured, the identification        parameter do have the correct reader identifier, and the        identification parameter is equal to Tag 3's second binding        handle, then Tag 3 responds to the ACK command and may no longer        be in sync with Reader E (as defined by the Gen2V2 protocol)    -   but if Tag 3's state is Open or Secured and if the        identification parameter do not have the correct reader        identifier, then Tag 3 ignores ACK command from Reader G, which        was on other objective of the present invention.

It should be noted that Tag 3 may receive other kinds of commands fromReader G:

-   -   if Tag 3 receives an inventory initiation command from Reader G,        Tag 3 shall switch from Reader E to Reader G and may be        subsequently inventoried and accessed by Reader G    -   if Tag 3 receives a Req_RN command from Reader G, then if Tag        3's state is Ready, Arbitrate, or Reply, or if the 16-bit number        comprised in said command does not match the expected value,        then Tag 3 shall ignore the Req_RN command. Otherwise, if the        16-bit value matches the expected value (which is highly        unlikely, though), then Tag3 shall respond to the Req_RN and may        no longer be in sync with Reader E.    -   if Tag 3 receives an Authenticate command from Reader G, then if        Tag 3's state is Ready, Arbitrate, Reply or Acknowledged, or if        the 16-bit value comprised in said command does not match the        expected binding handle value, then Tag 3 shall ignore said        command. Otherwise, if the 16-bit value matches the expected        value (which is highly unlikely), then Tag 3 shall respond to        the Authenticate command and may no longer be in sync with        Reader E.    -   if Tag 3 receives a Select command from Reader G, Tag 3 shall        switch from Reader E to Reader G and may be subsequently        inventoried and accessed by Reader G    -   if Tag 3 receives a Query command from Reader G, Tag 3 shall        switch from Reader E to Reader G and may be subsequently        inventoried and accessed by Reader G.    -   if Tag 3 receives a QueryRep or a QueryAdjust command from        Reader G, Tag 3 shall ignore said command as Tag 3's session is        different from the session that is mentioned in said command.

Although the exemplary embodiments have been described in detail withparticular reference to certain aspects thereof, it should be understoodthat the invention is capable of other embodiments and its details arecapable of modifications in various obvious respects. As is readilyapparent to those skilled in the art, variations and modifications canbe effected while remaining within spirit and scope of the invention.Accordingly, the foregoing disclosure, description, and FIGURES are forillustrative purposes only and do not in any way limit the invention,which is defined by the claims.

The invention claimed is:
 1. A method for providing asymmetricidentification and access with respect to a passive radio-frequency tagin a tag population, the passive tag being configured to receivecommands, including commands for acknowledging a tag, an ACK commandcomprising an identification parameter being a 16-bit number, the taghaving: a slot counter holding a value determining a point in aninventory round at which the tag may respond to a reader, a randomnumber generator or a pseudo-random number generator, configured togenerate a 16-bit value when the tag starts participating in theinventory round, and configured to generate another 16-bit value whenthe reader starts accessing the tag, a state that characterizes abehavior of the tag and a response to the commands of the reader, saidstate being part of a state list including Ready, which is a holdingstate for a tag not currently participating in the inventory round,Arbitrate, which is holding state for a tag currently participating inthe inventory round and whose slot counter holds a nonzero value, andOpen or secured, which are states a tag transitions to when a readerstarts accessing the tag, said method comprising the following steps,performed by the tag initially in the Ready state: receiving aninventory initiating command to initiate the inventory round, saidinventory initiating command comprising, as a parameter, a readeridentifier being a N-bit number, N being an integer in a range (1, 15),generating, with the random number generator or the pseudo-random numbergenerator, a 16-bit value being a first binding handle, said firstbinding handle comprising N juxtaposed bits forming the readeridentifier, backscattering the first binding handle, said first bindinghandle being received by the reader, receiving an access initiatingcommand to initiate a tag access, said access initiating command havingthe first binding handle as a parameter, generating, with the randomnumber generator or the pseudo-random number generator, a 16-bit valuebeing a second binding handle, said second binding handle comprising Njuxtaposed bits forming the reader identifier, said reader identifierbeing located within the second binding handle at a location similar toa location of the reader identifier within the first binding handle,backscattering the second binding handle, said second binding handlebeing received by the reader, transitioning to the Open or Securedstate, receiving an ACK command, comparing the reader identifier with acode formed by N bits of the identification parameter of said ACKcommand that are positioned at a location similar to the location of thereader identifier within the first or second binding handle, and whensaid code and said reader identifier differ, ignoring said ACK command,the tag having an inventoried flag for each session a reader may use toidentify the tag within the inventory round, said inventoried flaghaving a value indicating whether the tag may respond to the reader, theinventoried flag comprising at least two possible values, A or B, thefirst initiating command including a session number as a parameter, andone selected inventoried flag value of the at least two possible values,the method further comprising the following step, performed afterreceiving the inventory initiating command: setting the inventoried flagto the selected value for the session specified in the inventoryinitiating command, wherein only tags having the selected inventoriedflag value for the session number are inventoried, the first initiatingcommand only including a same inventoried flag for every inventoried tagduring a session, and wherein the at least other inventoried flag valueis never used during the session.
 2. The method according to claim 1,wherein when said code and said reader identifier match, comparing otherbits of the parameter with other bits of the second binding handle, andwhen said bits match, responding to the ACK command.
 3. The methodaccording to claim 1, wherein the state list comprises Reply, which is astate a tag currently participating in the inventory round transitionsto when the value in its slot counter becomes zero, the random numbergenerator or pseudo-random number generator being arranged to generate aQ-bit value when the tag starts participating in the inventory round, Qbeing an integer in the range, the inventory initiating commandcomprising Q as a slot-count parameter, the method further comprisingthe following step, performed after receiving the inventory initiatingcommand: generating, with the random number generator or thepseudo-random number generator, a Q-bit value and loading the Q-bitvalue into the slot counter, and transitioning to the Arbitrate state ifsaid Q-bit value is nonzero, or to the Reply state otherwise.
 4. Themethod according to claim 3, wherein the first binding handle and thesecond binding handle comprise Q juxtaposed bits different from the Njuxtaposed bits, said Q juxtaposed bits forming the Q-bit value, theQ-bit value being positioned within the first binding handle at alocation similar to a location of the Q-bit value within the secondbinding handle.
 5. The method according to claim 3, wherein Q is in arange 0 to
 5. 6. The method according to claim 4, wherein 16-N-Q bitsleft in the first and second binding handles are generated randomly bythe random number generator or the pseudo-random number generator. 7.The method according to claim 1, wherein the selected inventoried flagis always A for each session.
 8. The method according to claim 1,wherein the reader identifier is a 5-bit number, and N=5.